Reread a few times what Bill wrote earlier If you already have an account name or alias like JaneDoe@fastmail.fm, you automatically have these additional email addresses

JaneDoe+whatever@fastmail.fm (this is known as "plus addressing")

whatever@JaneDoe.fastmail.fm (this is known as "subdomain addressing")

some even more extreme examples which you should not need

You do not need aliases setup to be able to use these additional addresses. You get them for free.

To help your own understanding of this, send yourself a few emails (replacing JaneDoe@fastmial.fm with your own account name) to addresses like JaneDoe+GettingABetterIdea@fastmail.fm and NoLongerClueLess@JaneDoe.fastmail.fm
Try the specific address support is recommending you use to subscribe to newsletters as one of your tests. In most cases, all these messages will arrive in your Inbox.

Support is simply recommending that you use subdomain addressing to construct an email address for the purpose of subscribing to newsletters. My advice is to follow what they say exactly, without adding any additional steps you imagine could be necessary.